1. Field of the Invention
The present invention relates to a liquid crystal display apparatus wherein a pair of substrates holding a liquid crystal layer therebetween are attached to each other through a columnar spacer.
2. Description of the Prior Art
In recent years, a liquid crystal display apparatus, taking advantage of the features that it is both thin and low in power consumption, has been widely used for OA equipment such as a word processor and a personal computer, the portable information equipment such as an electronic dictionary or a single-unit video tape recorder having a liquid crystal monitor.
The liquid crystal display apparatus, which is not a self luminous display apparatus such as a CRT (Braun tube) or EL (electroluminescence), is generally divided into a reflection type and a transmission type. In the transmission-type liquid crystal display apparatus, light from a lighting unit (what is called back light) arranged on the back of a liquid crystal display panel is used for display, while the reflection-type liquid crystal display apparatus uses ambient light for display.
The transmission-type liquid crystal display apparatus uses the light from the back light for display, and therefore has the advantage that it is hardly affected by the brightness of the ambiance, and can display an image with a high contrast. The provision of the back light, however, leads to the disadvantage of a large power consumption. Incidentally, about 50% or more of the entire power consumed by the normal transmission-type liquid crystal display apparatus is consumed by the back light. Also, in a very bright operating environment (such as outdoor under the cloudless sky), the visibility is decreased. If the brightness of the back light is increased to maintain the visibility, on the other hand, the power consumption is further increased.
The reflection-type liquid crystal display apparatus, on the other hand, has no back light and therefore has the advantage of a very low power consumption. This makes the reflection-type liquid crystal display apparatus a suitable display for outdoor use. The reflection-type liquid crystal display apparatus, however, has the disadvantage that the visibility is low in a dark operating environment.
As a liquid crystal display apparatus compensating for the disadvantages of the transmission-type and reflection-type liquid crystal display apparatuses, a transmission/reflection-type liquid crystal display apparatus has been proposed in recent years as a liquid display apparatus having the dual display functions of transmission and reflection types.
In the transmission/reflection-type liquid crystal display apparatus, both a reflection pixel electrode for reflecting the ambient light and a transmission pixel electrode for transmitting the light from the back light are arranged in one pixel area (picture element area). In accordance with the operating environment (ambient brightness), therefore, display can be switched between transmission mode and reflection mode or both modes can be used for display at the same time.
The transmission/reflection-type liquid crystal display apparatus, therefore, has both the feature of the reflection-type liquid crystal display apparatus that the power consumption is low and the feature of the transmission-type liquid crystal display apparatus that the operation is not hardly affected by the ambient brightness and an image can be brightly displayed with a high contrast. Further, the transmission/reflection-type liquid crystal display apparatus offsets the disadvantage of the transmission-type liquid crystal display apparatus that the visibility is decreased in a very bright operating environment (such as outdoor under the cloudless sky).
A transmission/reflection-type liquid crystal display apparatus has been proposed, in which the transmissive region and the reflective region on the TFT substrate have different thickness of the birefringent liquid crystal layer to realize what is called a multigap (see Patent Document 1, for example). The liquid crystal display apparatus of this type is described briefly below.
FIG. 15 is a sectional view showing a general configuration of a liquid crystal display apparatus of multigap type. FIG. 16 is a plan view of a first substrate 101 formed with a TFT 106 of the liquid crystal display apparatus. In this liquid crystal display apparatus, a liquid crystal layer 103 is held between the first substrate 101 and the second substrate 102.
As shown in FIG. 16, a plurality of gate wires 104 and a plurality of source wires 105 are formed at right angles to each other on the first substrate 101. The portion defined by adjacent gate wires 104 and adjacent source wires 105 forms one pixel. At each intersection between the gate wire 104 and the source wire 105, a TFT 106 is formed as a switching element. The drain electrode 106a of the TFT 106 is arranged in such a manner as to cross a gate insulating film 108 covering an auxiliary capacitance electrode 107, as shown in FIG. 15.
Also, a transparent electrode 109 electrically connected to the drain electrode 106a is formed on the first substrate 101. A partial area of the transparent electrode 109 makes up a transmissive region for controlling the light transmission. The area other than the transmissive region on the first substrate 101 is formed with an inter-layer insulating film 110. A reflection electrode 111 is formed on the inter-layer insulating film 110. The area in which the reflection electrode 111 is formed constitutes a reflective region for controlling the reflection of the ambient light. An orientation film 112 is formed on the transparent electrode 109 and the reflection electrode 111.
Also, the boundary between the transmissive region and the reflective region of the inter-layer insulating film 110 is formed with a taper, and the reflection electrode 111 is formed also on the surface of the taper. When light is radiated on the reflection electrode 111 formed on the tapered portion, the reflected light is contained between the pair of the substrates and cannot exit, with the result that the light utilization efficiency is reduced. The area of the reflection electrode 111 formed on the tapered portion is not contributive to the display of the incident light and therefore, hereinafter referred to as a taper ineffective region.
On the other hand, a colored layer 113, an opposite electrode 114 and an orientation film 115 are stacked in this order on the second substrate 102.
In the liquid crystal display apparatus described above, the first substrate 101 and the second substrate 102 are attached to each other with a columnar spacer 116 arranged in the reflective region so that the thickness Td of the liquid crystal layer 103 in the transmissive region is about twice as large as the thickness Rd of the liquid crystal layer 103 in the reflective region. In this way, by realizing different cell gaps for the transmissive region and the reflective region, the phase difference (And) is substantially equalized between the transmissive region and the reflective region thereby to improve the display characteristic.
Incidentally, in order to realize the multigap described above, it is necessary to form a thin inter-layer insulating film 110 or remove the inter-layer insulating film 110 on the first substrate 101 in the transmissive region. Generally, the inter-layer insulating film 110 is formed of a photosensitive resin, and the thickness thereof is controlled by photolithography. Specifically, by adjusting the time of light exposure to the transmissive region, the thickness of the inter-layer insulating layer 110 to be removed at the time of development can be determined.
Also, a configuration in which a columnar spacer is arranged between a pair of substrates holding the liquid crystal layer therebetween is disclosed, for example, in Patent Document 2.
FIG. 17 is a sectional view showing a general configuration of the liquid crystal display apparatus described in Patent Document 2. In this liquid crystal apparatus, an auxiliary capacitance electrode 204 is formed in the same layer as the data wire through a gate insulating film 203 on the address wire 202 of the surface of a first substrate 201, and an auxiliary capacitor 205 is formed of the auxiliary capacitance electrode 204 and the address wire 202. A contact hole 207 is formed in the portion of the inter-layer insulating film 206 formed with the auxiliary capacitor 205. The auxiliary capacitance electrode 204 and the pixel electrode 208 formed on the inter-layer insulating film 206 are electrically connected to each other by the contact hole 207.
On the other hand, a color filter 212 is formed on the one hand and a columnar spacer 213 is formed by a stack of colored layers on the second substrate 211. The first substrate 201 and the second substrate 211 are attached to each other through a liquid crystal layer 214 so that the forward end of the columnar spacer 213 is arranged in the contact hole 207.
In the liquid crystal display apparatus described in Patent Document 3, a columnar spacer for holding the pair of substrates at a predetermined interval to hold the liquid crystal is formed of a resin material blackened by nonelectrolyte plating thereby to prevent the irregular reflection of the light and thus to prevent the reduction in contrast.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2002-72220    [Patent Document 2] Japanese Unexamined Patent Publication No. 10-96955    [Patent Document 3] Japanese Unexamined Patent Publication No. 2002-174817
The size of the columnar spacer 116 fails to be sufficiently studied in Patent Document 1. When the attachment between the first substrate 101 and the second substrate 102 is displaced, therefore, the columnar spacer 116 may fall in the contact hole (in the case of FIG. 15, the transmissive region portion free of the inter-layer insulating film 110), thereby posing the problem that a desired cell gap cannot be positively secured.
Similarly in the case where the columnar spacer 213 is located in the contact hole 207 as described in Patent Document 2, the displacement between the first substrate 201 and the second substrate 211 attached to each other and hence the displacement of the coordinates between the columnar spacer 213 and the contact hole 207 would make it impossible to contain the columnar spacer 213 in the contact hole 207 and thus to secure the desired thickness of the liquid crystal layer 214.
For example, assume that the attachment accuracy between the first substrate 201 and the second substrate 211 (attachment margin A) is 5 μm in horizontal and vertical directions and the diameter of the columnar spacer 213 is 12 μm. In order to contain the columnar spacer 213 securely in the contact hole 207, the inner diameter of the bottom portion of the contact hole 207 is required to be at least 22 μm.
In the case where the contact hole 207 is formed in the reflective region, for example, the liquid crystal layer 214 in the area formed with the contact hole 207 has no desired thickness and constitutes an ineffective region not contributive to display. This ineffective region is increased by the attachment margin A, thereby posing the problem of a reduced open area ratio. Also, in the attachment margin A, i.e. the region of at least 5 μm around the columnar spacer 213, an area remains where the thickness of the liquid layer 214 is larger than the desired value, thereby probably causing a reduced contrast (the particular area, even if contributive to the display, adversely affects the display).